. Chronic kidney disease (CKD) and diabetes mellitus (DM) contribute significantly to cardiovascular disease (CVD) and mortality, with prevalence increasing. The evolving demographic of myocardial infarction (MI) patients, influenced by sedentary lifestyles and advanced medical care, lacks understanding regarding the interplay of CKD, DM, age, and post-MI mortality. This study aims to address this gap by evaluating the long-term impact of CKD and DM on post-MI mortality across age groups. . A retrospective cohort study utilized data from the Estonian Myocardial Infarction Registry (EMIR), Estonian Population Register (EPR), and six major hospitals in Estonia, covering AMI hospitalizations from 2012 to 2019. Statistical analyses included Cox proportional hazards regression models and Kaplan-Meier's curves. . Analysis of 17,085 MI patients revealed age-dependent associations between renal function and mortality. In patients <65 years, even minor decreases in renal function increased both short-term (HR 2.79, 95% CI 1.71-4.55) and long-term (HR 1.24, 95% CI 1.05-1.47) mortality. Mortality significantly increased in patients >80 years only below an estimated glomerular filtration rate (eGFR) of 44 ml/min/1.73 m. Newly diagnosed DM patients exhibited higher mortality rates (average HR 1.53, 95% CI 1.45-1.62), while pre-DM did not significantly differ from non-DM patients across all age groups. The DM-renal failure interaction did not significantly influence mortality. . An age-dependent association between eGFR and post-MI outcomes emphasizes the need for personalized therapeutic approaches considering age-specific eGFR thresholds and comorbidities to optimize patient management.